It is well known that silver and copper have inhibitory effects on pathogenic bacteria growth, typically represented by Escherichia coli and salmonella, and acts for preventing food poisoning caused by pathogenic bacteria.
Recently, materials provided with inhibitory effects on bacterial growth (hereinafter referred to as antibacterial characteristics) by using these metals have been proposed. In Japanese Unexamined Patent Application Publication No. 8-49085, for example, a stainless steel sheet having superior antibacterial properties is disclosed; on the surfaces of the stainless steel sheet, metal layers or alloy layers of chromium, titanium, nickel, iron and the like containing silver and/or copper are formed by magnetic sputtering. In this steel sheet, metal layers or alloy layers formed containing 19 to 60 percent by weight of silver is preferable.
In Japanese Unexamined Patent Application Publication No. 8-156175, a steel sheet coated by pigments containing silver, which can suppress bacterial growth, is proposed.
However, in the methods described above for forming the metal layers or the alloy layers on the steel sheet surfaces including metals having the antibacterial properties, and in the methods for coating the pigments including the metals having the antibacterial properties, the layers including the metals having the antibacterial properties are stripped or removed by drawing and surface polishing, and the problems are that the effects cannot thereby be anticipated. In applications, such as steel sheets used for the interiors of washing machines, which are continually abraded, and steel sheets used for kitchens which are frequently scrubbed for cleaning, the problem is that the antibacterial properties do not last over long periods of time. In the methods described above, additional steps for manufacturing the steel sheets are required to form coating layers, metal layers, and alloy layers. In addition, when steel sheets are made thinner, since the amounts of coating, metal layers, and alloy layers per weight increase concomitant with an increase of surface area per weight, there is a disadvantage in terms of cost.
In order to solve the problems described above, there have been proposed austenitic stainless steel enhancing antibacterial properties by adding 1.1 to 3.5 percent by weight of copper as disclosed in Japanese Unexamined Patent Application Publication No. 8-104953; martensitic stainless steel enhancing antibacterial properties by adding 0.3 to 5 percent by weight of copper as disclosed in Japanese Unexamined Patent Application Publication No. 8-104952; and ferritic stainless steel enhancing antibacterial properties by adding 0.4 to 3.0 percent by weight of copper as disclosed in Japanese Unexamined Patent Application Publication No. 9-170053.
However, in the technologies disclosed in Japanese Unexamined Patent Application Publications Nos. 8-104953, 8-104952, and 9-170053, copper ions must leach from the surfaces of the steel sheets to produce the antibacterial properties. The leaching of copper in ionic form is due to breakage of passivation layers at the leaching points, and corrosion resistance is therefore seriously degraded, even though antibacterial properties are improved. Accordingly, it is difficult for stainless steel having copper therein to have antibacterial properties and corrosion resistance at the same time.
Objects of the present invention are to provide stainless steel and a manufacturing method therefor by solving the problems in the conventional technologies. The stainless steel of the present invention has superior antibacterial properties and corrosion resistance, and continues to have superior antibacterial properties even after surface processing is performed, such as polishing.